1. Field of the Invention
The present invention relates to an image taking lens which is suitable for a compact imaging apparatus, an optical sensor, a module camera of a mobile phone, a web camera, and so on which use a solid-state imaging device such as a high resolution CCD or CMOS.
2. Description of the Related Art
In recent years, various kinds of imaging apparatuses using a solid-state imaging device such as a CCD or a CMOS are spread in popular. Following to miniaturization and technical advantageous of the imaging device, it is required to supply a compact, lightweight image taking lens having a high optical performance.
For downsizing and weight-trimming of the image taking lens, it is conventionally proposed to configure the image taking lens with a lens system having one piece of lens, or a lens system having two pieces of lenses. Although such lens systems are advantageous for downsizing and weight-trimming, it is well known that these lens systems have a difficulty to correct aberrations such as curvature of field, and thus, splendid optical performance cannot be expected. Therefore, the image taking lens having a splendid optical performance is generally configured by more than two pieces of lenses.
Even in the image taking lens configured by three pieces of lenses, and even when all surfaces of the three pieces of lenses are shaped aspherical, it may be insufficient to correct aberrations in peripheral area when it is used for a high resolution CCD having more than two million pixels, for example. Therefore, an image taking lens configured by four pieces of lenses, by which correction of aberrations becomes easier and a high optical performance can be obtained than the image taking lens configured by three pieces of lenses, is developed, and various kinds of image taking lenses configured by four pieces of lenses are proposed.
JPA 2002-365529 discloses a conventional image taking lens comprising an aperture, a first lens having a positive power of biconvexity configuration, a second lens having a negative power of biconcavity configuration, a third lens having a positive power and in which a convex surface is disposed to face an image, and a fourth lens having a negative power and in which a convex surface is disposed to face an object, which are arranged in this order from the object. According to such a configuration, the first lens and the second lens are designed to have strong powers to downsize the image taking lens. Since a lens element of high refractive index is used for the second lens, it, however, is difficult to take a balanced allocation of power with other lens elements. Consequently, a focal length of entire image taking lens cannot be shortened or correction of aberrations may be insufficient.
JPA 2002-365531 discloses another conventional image taking lens comprising an aperture, a first lens having a positive power of biconvexity configuration, a second lens having a negative power of biconcavity configuration, a third lens having a positive power and in which a convex surface is disposed to face an image, and a fourth lens having a negative power and in which a convex surface is disposed to face an object, which are arranged in this order from the object. According to such a configuration, the first lens and the second lens are designed to have strong powers to downsize the image taking lens. Since an index of refraction of the second lens is relatively small, it may be difficult to take balanced power allocation in entire of the image taking lens. Consequently, a focal length of entire image taking lens cannot be shortened or correction of aberrations may be insufficient.
JPA 2007-17984 (US 2007-0008625 A1) discloses still another conventional image taking lens comprising a first lens group having a positive power, a second lens group having a negative power, a third lens having a positive power or a negative power, and a fourth lens group having a positive power or a negative power. Since a lens element of high refractive index is used for the second lens group, it, however, is difficult to take a balanced allocation of power with other lens elements, especially the first lens group. Therefore, it is difficult to satisfy both of downsizing the image taking lens and realizing a high optical performance. In addition, this document discloses an example of an image taking lens having a membrane filter of an optical filter to stop a part of light beams in between the lens groups. It causes the increase of a number of optical elements and processes of production.